It is known, in the field of orthopedic prostheses, to make an insert with a semi-spherical cavity which functions as a positioning and rotation seating for the head of a femoral prosthesis.
The insert, in turn, is able to be disposed inside a mating casing or seating, shaped like a cone or truncated cone, made of osteo-compatible material, for example based on titanium or cobalt, of an acetabular cup of the hip. Both the insert and the acetabular cup are normally axial-symmetrical with respect to a common axis of symmetry.
The insert is usually made of cobalt-based metal, such as an alloy of cobalt-chrome-molybdenum, or of ceramic, or also of polyethylene, which is lighter and more economical.
In the case of polyethylene inserts, however, there may be an unwanted tendency to a relative rotation of the insert and the acetabular cup, with respect to the common axis of symmetry. This rotation between the insert and the acetabular cup is due both to the common circular geometry of the two components, and also to the difference in material of which they are made, since the acetabular cup is usually made of metal. Such rotation must absolutely be prevented, so as not to cause serious damage to the patient's articulation, such as dislocation or other.
This problem is found, however, every time there is a tendency to reciprocal rotation of the insert and the relative acetabular cup.
Purpose of the present invention is to achieve an insert that, once disposed in the casing of the acetabular cup, will maintain during normal use the position determined during the operation to insert the prosthesis and the acetabular cup, and that will not rotate with respect to the acetabular cup, and in particular will not rotate with respect to the common axis of symmetry.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.